The purpose of this research project is to determine the site of certain long-term alterations occurring in the mammalian spinal cord and to analyze the alterations at the cellular level. The proposed studies are concerned with two subjects: (1) alterations of synaptic efficacy following use or disuse of the synapses; and (2) the factors involved in differentiation of spinal motoneurons into 'phasic' (fast-alpha) and 'tonic' (slow-alpha) types. For these subjects, four inter-related questions will be tested: (a) Is synaptic efficacy in motoneurons modified by prolonged presynaptic stimulation or by prolonged cessation of presynaptic activity? (b) Does synaptic efficacy in motoneurons progressively increase with usage of central synapses during postnatal development? (c) Is muscle differentiation into fast- (pale muscle) and slow-contracting (red muscle) types associated with motoneuron differentiation into 'phasic' and 'tonic' types? (d) Is motoneuron differentiation into 'phasic' and 'tonic' types modified by denervation and reinnervation of the muscles? The methods to be employed are exclusively electrophysiological, using the techniques of intracellular recording. It is anticipated that the results of these studies will contribute to our understanding of the plastic and trophic properties in the central nervous system.